The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source, many of which relate to environmental impacts of burning nitrogen-, sulfur-, and aromatic-containing fuels. Alternative sources for developing combustible liquid fuels are desirable. One such source that is in abundant supply is natural gas. The conversion of natural gas to combustible liquid fuel typically involves converting the natural gas, which is mostly methane, to synthesis gas, also called syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they typically do not contain appreciable amounts of nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have less environmental impact than conventional petroleum-based fuels. Fischer-Tropsch synthesis is a preferred means for converting syngas to higher molecular weight hydrocarbon products.
The present invention is directed toward a process for reducing the impact of by-products produced during a Fischer-Tropsch (FT) synthesis process. Carbon oxides (e.g. carbon monoxide and carbon dioxide) are created both in the production of syngas and as a result of a Fischer-Tropsch hydrocarbon synthesis process. While CO is a reactant in the Fischer-Tropsch process, both CO and CO2 are present in the Fischer-Tropsch products. Light Fischer-Tropsch hydrocarbon fractions recovered especially from slurry bed Fischer-Tropsch reactors contain significant quantities of carbon oxides. These carbon oxides, when allowed to remain in the Fischer-Tropsch light hydrocarbon fraction, can consume significant amounts of H2 during downstream hydroprocessing; reduce H2 partial pressure and cause rapid catalyst fouling during downstream hydroprocessing (due to the high amounts of diluent carbon oxide gases in recycle gas systems); acidify reactor effluents, especially water (which requires expensive equipment metallurgy); and foul catalytic hydrogenation metals, especially noble metals. Therefore, an object of this invention is to remove carbon oxides and other components at appropriate points in the processing.
The present process removes methane as well as carbon oxides, which reduces equipment costs in down stream processing and reduces the diluent effect of methane in recycle gas streams. The present process provides a method for pre-conditioning Fischer-Tropsch light products in general prior to upgrading, and particularly provides a process for pre-conditioning light Fischer-Tropsch slurry bed reaction synthesis hydrocarbon products prior to upgrading, the advantages of which will be apparent.
Embodiments of the present invention are directed toward a process for pre-conditioning light Fischer-Tropsch slurry bed reactor synthesis hydrocarbon products prior to upgrading in a hydroprocessing unit. According to these embodiments, certain components of the light hydrocarbon product are removed since they are detrimental to downstream hydroprocessing equipment, catalysts, and operating economics. Embodiments of the present invention are directed toward a method for removing and optionally recovering CO2 contained in products from a Fischer-Tropsch synthesis process. An exemplary process comprises:
(a) isolating a light fraction and a heavy fraction from a Fischer-Tropsch synthesis product;
(b) separating the light fraction into a carbon oxide-containing gaseous fraction and a treated light fraction;
(c) subjecting the treated light fraction to hydroprocessing step to yield a light effluent; and
(d) recovering hydrocarbons, liquid fuels, and other useful products from the effluent of the hydroprocessing step.
In this exemplary embodiment of the process, carbon oxides (i.e. CO2 and CO) contained in a light fraction from the Fischer-Tropsch synthesis process are separated from the hydrocarbons in the light fraction, preferably by stripping, distillation or fractionation. A gaseous stream from the separation step includes greater than 75% v/v, preferably greater than 85% v/v, and more preferably greater than 95% v/v of the of the carbon oxides contained in the light fraction. This gaseous stream may include minor amount of hydrocarbons, where methane is typically the dominant hydrocarbon component. Preferably, the gaseous fraction comprises greater than 50 percent by weight carbon oxides. A treated light fraction from the separation step (b) contains trace amounts of carbon oxides, preferably less than 500 ppm, more preferably less than 100 ppm, and most preferably less than 50 ppm. Preferably, the gaseous stream from the separation step contains less than 15 percent by weight C2-C4 hydrocarbons.
At least a portion of the treated light fraction recovered from the separating step (b) may be further converted to meet the needs of a particular process. Exemplary additional processing steps may include dehydrogenation to produce aromatics and/or gasoline, thermal or steam cracking to produce olefins such as ethylene, hydrocracking for molecular weight reduction, hydrotreating to remove olefins and oxygenates, hydroisomerization to form low pour point isoparaffins, catalytic dewaxing for wax removal and pour point reduction, and hydrofinishing for improving product stability. In a preferred embodiment, a light fraction having an distillation endpoint (EP) in the range of 650-750xc2x0 F. and a heavy fraction having an initial boiling point (IBP) in the range of 650-750xc2x0 F. are recovered from the Fischer-Tropsch synthesis process. These products, either in combination or singly, may be hydroprocessed to prepare fuels and/or lubricating oil base stocks.
Among other factors, embodiments of the present invention are based on the discovery of a process for treating Fischer-Tropsch synthesis products to remove carbon oxides to very low levels with little or no loss of hydrocarbon product. The process effectively separates the carbon oxides from the light hydrocarbons, the latter of which are then further processed for making or blending into fuels and, optionally, lubricating oils. Advantages of the present process include improved yields of valuable products from a Fischer-Tropsch synthesis process. Embodiments of the present invention provide methods for removing and optionally recovering carbon oxides, particularly CO2, in the form of a substantially purified stream for disposal if desired.